The present invention relates to a method of relieving residual stress in a metal pipe.
U.S. Pat. No. 3,986,634 discloses a method of relieving residual stress in a metal pipe, in which the metal pipe is thermally treated together with a mandrel which has a thermal expansion coefficient greater than the pipe. This treatment is generally referred to as "thermal sizing treatment". This method, however, cannot be applied satisfactorily to a long pipe because there is a risk that the inner surface of the pipe may be damaged by the mandrel during insertion of the same, as a result of deformation of the pipe incurred during press work, welding or other work conducted prior to the thermal sizing treatment. Additionally, a long time period is required for the insertion and withdrawal of the mandrel, as well as for the removal of the scratching or other defects in the inner surface of the pipe due to the insertion of the mandrel.